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SEISMIC BRIDGE PROBLEM
- To: "Sseaint Org" <seaint(--nospam--at)seaint.org>
- Subject: SEISMIC BRIDGE PROBLEM
- From: "gregory szuladzinski" <ggg(--nospam--at)bigpond.net.au>
- Date: Tue, 12 Jul 2011 18:55:42 +1000
Structural dynamics is just that, no matter what
structure, be it a bridge or something altogether different. As I remember, the
first time I had to resolve the problem of how to include a gap in a system was
in 1975. It is coming back, now and then, like a 5-year tide.
Your total restraint at each end of the bridge is made up
of soil, abutment, rubber pad and a gap.
- Both the pad and
the gap are N/L elements. That means that if you insist on the spectrum
response method, you have to develop an equivalent spring, representing all
- That linear spring
is forcing-dependent, valid for a specific forcing or a specific response
- The method of
finding the equivalent k is a bit round-about, but quite rational.
- The reason for
concern is when a method used makes the seismic effects smaller, than they
really should be. With a rubber pad it can go either way, i.e. you can
artificially increase or decrease the response.
- There is less doubt
with a gap. If you choose to ignore it, for computational convenience,
you are artificially
increasing the natural frequency. This usually results in decreasing of the
computed response, which is unconservative and unsafe.
- Unless the people
who did those analyses know how to handle the two nonlinearities, the
inclusion of only the linear stiffness component is unsafe.
- One of the ways to
avoid creating equivalent springs is to conduct a time-dependent N/L
simulation, but that is luxury for most engineering offices.
- If you wanted to
see many examples of practical handling of nonlinearities, you could borrow
from a library my first book, entitled “Dynamics of structures and machinery.
Problems and solutions”, Wiley 1982.
- I am not in a
position to comment on political and legal aspects of the
Still, I believe that I confirmed your suspicion with
regard to the quality of the results.